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Estimating the magnitudes of potential large earthquakes and associated tsunami hazards along a subduction zone is important for risk evaluation. Coastal boulder deposits can serve as valuable geological indicators in intertidal zones with high wave velocity, even though finer sediments are generally washed away. This report presents a new method to constrain the maximum possible sizes of paleo-earthquakes and tsunamis using boulders of storm wave origin as a constraint in numerical simulations. The fundamental assumption is that presently observed storm-derived boulders have not moved a great distance by past tsunami waves. This illustrative case study uses the sizes and spatial distributions of storm wave boulders on the reef at Kudaka Island, Okinawa Prefecture, which faces the Ryukyu Trench. In all, 47 tsunami scenarios with different locations and fault parameters were assumed. Results show that tsunami waves with a maximum water level of 2.2 m and maximum flow speed of 1.5 m/s at a beach of Kudaka Island did not strike the island. Under this condition, earthquakes of Mw≥8.3 are unlikely to have occurred in the central Ryukyu Trench if one obeys an empirical scaling law for subduction earthquakes or ≥2 m of maximum possible slip near the trench if the fault size is fixed as 100 km × 50 km. Those tsunami and earthquake sizes could not have been generated during this period if one assumes that the reef of Kudaka Island and emplacement of storm boulders began more than 3500 years ago. This finding implies that the seismogenic mechanism of the Ryukyu Trench and the risk of a giant earthquake differ from those of other trenches such as the Japan Trench and the Chilean Trench, where giant earthquakes of Mw≥9.0 have occurred repeatedly during the past. The proposed method would enable researchers to estimate the maximum possible sizes of the paleo-earthquakes and tsunami waves in any region, even in regions with a vague or unknown history of past events because of a lack of sandy tsunami deposits or seismic records.
Typhoons and associated storm waves in the northwestern Pacific Ocean commonly cause coastal disasters. The possibility remains that an even stronger typhoon than the strongest one observed to date might have occurred before. The development of a method to estimate a maximum intensity of past typhoons over thousands of years is important for paleoclimatology, paleoceanography and disaster prevention. Numerous storm wave boulders exist on reefs in the Ryukyu Islands, Japan, which have been deposited to their present position by the cumulative effects of the past storm waves. These boulders can be used as proxies for the hydrodynamic conditions of the largest waves from past events. Here, we present numerical computations for storm waves and boulder transport with the boulder distribution as a constraint factor to estimate the maximum intensities of storm waves and their causative typhoon events over the past 3500 years. Though the intensities of the maximum estimated waves and associated typhoon events were slightly stronger than those recorded over the past ~70 years in the Ryukyu Islands, our results suggest that no abnormally intense typhoon has struck the Ryukyu Islands in the past 3500 years. The potential impact from tsunamis remains uncertain; however, our results are meteorologically reasonable.
Coastal boulder deposits (CBD), transported by waves at elevations above sea level and substantial distances inland, are markers for marine incursions. Whether they are tsunami or storm deposits can be difficult to determine, but this is of critical importance because of the role that CBD play in coastal hazard analysis. Equations from seminal work by Nott (1997), here referred to as the Nott Approach, are commonly employed to calculate nominal wave heights from boulder masses as a means to discriminate between emplacement mechanisms. Systematic review shows that this approach is based on assumptions that are not securely founded and that direct relationships cannot be established between boulder measurements and wave heights. A test using an unprecedented dataset of boulders moved by storm waves (with associated sea-state data) shows a lack of agreement between calculations and actual wave heights. The equations return unrealistically large heights, many of which greatly exceed sea states occurring during the boulder-moving storms. This underscores the finding that Nott-Approach wave-height calculations are unreliable. The result is general, because although the field data come from one region (the Aran Islands, Ireland), they represent a wide range of boulder masses and topographic settings and present a valid test of hydrodynamic equations. This analysis demonstrates that Nott Approach equations are incapable of distinguishing storm waves from tsunami transport and that wave heights hindcast from boulder masses are not meaningful. Current hydrodynamic understanding does not permit reliable computation of wave height from boulder measurements. A combination of field, numerical, and experimental approaches is required to quantify relationships between wave power and mass transport onshore. Many CBD interpreted as tsunami deposits based on Nott-Approach analysis may in fact have been emplaced during storms and should therefore be re-evaluated. This is especially important for CBD that have been incorporated into long-term coastal risk assessments, which are compromised if the CBD are misinterpreted. CBD dynamics can be better determined from a combination of detailed field measurements, modeling, and experiments. A clearer understanding of emplacement mechanisms will result in more reliable hazard analysis.
Sea level indicators, such as tidal notches and beachrocks, may provide valuable information for the relative sea level (RSL) changes of an area. The study area, Okinawa, belongs to the Ryukyu Islands, Japan (Pacific Ocean), forming the emerged part of an active island arc, where the Philippine Sea plate is subducting beneath the Asian continent. Evidence of emergence has been noted by various studies. Beachrocks have also been studied, however, detailed examinations of their spatial extent and cement characteristics has not been accomplished. The purpose of this study is to discuss the RSL evolution in Okinawa through the re-evaluation of reported sea level indicators, with additional observations of beachrocks and notches and RSL predictions. Our findings suggest that the majority of Okinawa beachrocks have formed in the intertidal zone. Although the vertical uncertainty of the produced SLIPs is relatively large, there is a good agreement between the different types of sea level indicators. Comparisons with RSL predictions as well as the presence of uplifted notches further suggest that Okinawa island is generally characterized by an uplift trend, which is larger in its southern part.
The giant 1771 Yaeyama tsunami occurred in the southwestern part of the Ryukyu Arc, a region on an obliquely subducting plate boundary, which shows no direct evidence of inter-plate coupling. Studies of tsunami boulders and deposits suggest that the recurrence interval of comparably giant tsunamis is roughly 500 to 1000 years. Tsunami source models, which include either slip on a shallow plate boundary or active faulting plus a landslide on the overriding plate, are controversial because of inconsistencies in the geophysical and geological data. We discovered a seafloor depression that is approximately 30 km wide and 80 km long extending in the ESE-WNW direction. This depression is accompanied by a seaward bulge on the accretionary prism along the Ryukyu Trench, which is based on detailed bathymetric data and interpreted to be the result of accretionary prism collapse and seaward displacement by rotational slide. A simple tsunami simulation shows that the slide is a plausible source of the 1771 tsunami. We propose a collapse model, in which the accretionary prism remained over-steepened as strike-slip faulting removed the prism toe. Our model indicates that some oblique subduction zones are capable of generating giant tsunamis regardless of weak or strong coupling.
Subduction zone megathrust faults host Earth's largest earthquakes, along with multitudes of smaller events that contribute to plate convergence. An understanding of the faulting behavior of megathrusts is central to seismic and tsunami hazard assessment around subduction zone margins. Cumulative sliding displacement across each megathrust, which extends from the trench to the downdip transition to interplate ductile deformation, is accommodated by a combination of rapid stick-slip earthquakes, episodic slow-slip events, and quasi-static creep. Megathrust faults have heterogeneous frictional properties that contribute to earthquake diversity, which is considered here in terms of regional variations in maximum recorded magnitudes, Gutenberg- Richter b values, earthquake productivity, and cumulative seismic moment depth distributions for the major subduction zones. Great earthquakes on megathrusts occur in irregular cycles of interseismic strain accumulation, foreshock activity, main-shock rupture, postseismic slip, viscoelastic relaxation, and fault healing, with all stages now being captured by geophysical monitoring. Observations of depth-dependent radiation characteristics, large earthquake slip distributions, variations in rupture velocities, radiated energy and stress drop, and relationships to aftershock distributions and afterslip are discussed. Seismic sequences for very large events have some degree of regularity within subduction zone segments, but this can be complicated by supercycles of intermittent huge ruptures that traverse segment boundaries. Factors influencing variability of large megathrust ruptures, such as large-scale plate structure and kinematics, presence of sediments and fluids, lower-plate bathymetric roughness, and upper-plate structure, are discussed. The diversity of megathrust failure processes presents a suite of natural hazards, including earthquake shaking, submarine slumping, and tsunami generation. Improved monitoring of the offshore environment is needed to better quantify and mitigate the threats posed by megathrust earthquakes globally.
This study investigated the activity and distribution of low-frequency earthquakes (LFEs) accompanying the very low frequency earthquakes (VLFEs) in the central and southern Ryukyu Trench. This investigation was based on short-period seismometer waveforms obtained by the Japan Meteorological Agency from April 2004 to December 2015. The LFEs were detected using the Envelope Correlation Method, and the hypocenter locations were established using the arrival time difference of the envelope. The arrival times of the P- and S-phases were selected for events in which conspicuous P and S arrivals were observed and their hypocenters were determined. The results showed that LFEs are distributed 30–50 km from the trench axis in the Okinawa and Yaeyama areas. These areas correspond to a slab depth of 12–25 km. The LFEs and VLFEs occurred in association with slow slip events (SSEs) in the Okinawa and Yaeyama areas, indicating that they are induced by SSEs in the Ryukyu Trench. Moreover, the SSEs, LFE–VLFEs, and thrust-type ordinary earthquakes exhibit separate distributions. This suggests change in the frictional condition at the slab depth of 12–25 km along the trench axis in the Ryukyu subduction zone. In the southern Ryukyu Trench, LFEs occur approximately 50 km from the SSE faults, suggesting that SSEs trigger the LFEs near the southern Ryukyu Trench.Open image in new windowGraphical abstract.
Extraordinary marine inundation scattered clasts southward on the island of Anegada, 120 km south of the Puerto Rico Trench, sometime between 1200 and 1480 calibrated years (cal yr) CE. Many of these clasts were likely derived from a fringing reef and from the sandy flat that separates the reef from the island's north shore. The scattered clasts include no fewer than 200 coral boulders, mapped herein for the first time and mainly found hundreds of meters inland. Many of these are complete colonies of the brain coral Diploria strigosa. Other coral species represented include Orbicella (formerly Montastraea) annularis, Porites astreoides, and Acropora palmata. Associated bioclastic carbonate sand locally contains articulated cobble-size valves of the lucine Codakia orbicularis and entire conch shells of Strombus gigas, mollusks that still inhabit the sandy shallows between the island's north shore and a fringing reef beyond. Imbricated limestone slabs are clustered near some of the coral boulders. In addition, fields of scattered limestone boulders and cobbles near sea level extend mainly southward from limestone sources as much as 1 km inland. Radiocarbon ages have been obtained from 27 coral clasts, 8 lucine valves, and 3 conch shells. All these additional ages predate 1500 cal yr CE, all but 2 are in the range 1000-1500 cal yr CE, and 16 of 22 brain coral ages cluster in the range 1200-1480 cal yr CE. The event marked by these coral and mollusk clasts likely occurred in the last centuries before Columbus (before 1492 CE). The pre-Columbian deposits surpass Anegada's previously reported evidence for extreme waves in post-Columbian time. The coarsest of the modern storm deposits consist of coral rubble that lines the north shore and sandy fans on the south shore; neither of these storm deposits extends more than 50 m inland. More extensive overwash, perhaps by the 1755 Lisbon tsunami, is marked primarily by a sheet of sand and shells found mainly below sea level beneath the floors of modern salt ponds. This sheet extends more than 1 km southward from the north shore and dates to the interval 1650-1800 cal yr CE. Unlike the pre-Columbian deposits, it lacks coarse clasts from the reef or reef flat; its shell assemblage is instead dominated by cerithid gastropods that were merely stirred up from a marine pond in the island's interior. In their inland extent and clustered pre-Columbian ages, the coral clasts and associated deposits suggest extreme waves unrivaled in recent millennia at Anegada. Bioclastic sand coats limestone 4 m above sea level in areas 0.7 and 1.3 km from the north shore. A coral boulder of nearly 1 m3 is 3 km from the north shore by way of an unvegetated path near sea level. As currently understood, the extreme flooding evidenced by these and other clasts represents either an extraordinary storm or a tsunami of nearby origin. The storm would need to have produced tsunami-like bores similar to those of 2013 Typhoon Haiyan in the Philippines. Normal faults and a thrust fault provide nearby tsunami sources along the eastern Puerto Rico Trench.
It has been recognized that even weakly coupled subduction zones may cause large interplate earthquakes leading to destructive tsunamis. The Ryukyu Trench is one of the best fields to study this phenomenon, since various slow earthquakes and tsunamis have occurred; yet the fault structure and seismic activity there are poorly constrained. Here we present seismological evidence from marine observation for megathrust faults and low-frequency earthquakes (LFEs). On the basis of passive observation we find LFEs occur at 15-18 km depths along the plate interface and their distribution seems to bridge the gap between the shallow tsunamigenic zone and the deep slow slip region. This suggests that the southern Ryukyu Trench is dominated by slow earthquakes at any depths and lacks a typical locked zone. The plate interface is overlaid by a low-velocity wedge and is accompanied by polarity reversals of seismic reflections, indicating fluids exist at various depths along the plate interface.
The recording on high-resolution broadband seismic networks of several great interface subduction earthquakes during the last decade provide an excellent opportunity to extend source-scaling relations to very large magnitudes and to place constraints on the potential range of source parameters for these events. At present, there is a wide range of uncertainty in the median rupture areas predicted for any given seismic moment by current relationships between magnitude and rupture area for subduction interface earthquakes. Our goal is to develop an updated set of earthquake source-scaling relations that will reduce this current large degree of epistemic uncertainty and improve the accuracy of seismic-hazard analysis and the prediction of the strong-motion characteristics and tsunamis of future subduction earthquakes. To achieve this goal, we compiled a database including slip models of interface earthquakes that occurred worldwide with moment magnitudes ranging from M 6.75-9.1. We characterized the seismic sources based on well-established criteria to estimate the asperity areas as well as the average slip on the faults, and we used these parameters to compute an updated set of magnitude scaling relations of the various characteristics of the fault. Additionally, we followed an alternative approach to quantifying slip models for use in developing characteristic slip models of future earthquakes. This involved analyzing the 2D Fourier transforms of the slip functions in the compiled database and deriving a wavenumber spectral model of the slip distribution.
Focuses on the relationships between time, space and geomorphic environments of the Holocene reef-flat formation. The discussion is based on nine core logs drilled in the Holocene emerged reef-flat of Nishimezaki Kume Island, the Central Ryukyus. The apparent sea-level curve during the period from 7500 to 2000 Y.BP can be divided into three phases on the basis of sea-level rise rates. The formational process of the reef-flat topography could also be divided into three phases. In contrast to the sea-level rise rate and the wave direction controlling unilaterally the reef accretion processes, topography is both a result of reef accretion and a factor which controls the subsequent reef accretion. -from Authors
On 1 April 2014, Northern Chile was struck by a magnitude 8.1 earthquake following a protracted series of foreshocks. The Integrated Plate Boundary Observatory Chile monitored the entire sequence of events, providing unprecedented resolution of the build-up to the main event and its rupture evolution. Here we show that the Iquique earthquake broke a central fraction of the so-called northern Chile seismic gap, the last major segment of the South American plate boundary that had not ruptured in the past century. Since July 2013 three seismic clusters, each lasting a few weeks, hit this part of the plate boundary with earthquakes of increasing peak magnitudes. Starting with the second cluster, geodetic observations show surface displacements that can be associated with slip on the plate interface. These seismic clusters and their slip transients occupied a part of the plate interface that was transitional between a fully locked and a creeping portion. Leading up to this earthquake, the b value of the foreshocks gradually decreased during the years before the earthquake, reversing its trend a few days before the Iquique earthquake. The mainshock finally nucleated at the northern end of the foreshock area, which skirted a locked patch, and ruptured mainly downdip towards higher locking. Peak slip was attained immediately downdip of the foreshock region and at the margin of the locked patch. We conclude that gradual weakening of the central part of the seismic gap accentuated by the foreshock activity in a zone of intermediate seismic coupling was instrumental in causing final failure, distinguishing the Iquique earthquake from most great earthquakes. Finally, only one-third of the gap was broken and the remaining locked segments now pose a significant, increased seismic hazard with the potential to host an earthquake with a magnitude of >8.5.
 The nature of seismic coupling for many of the world's subduction zones has been reevaluated. Geodetic estimates of seismic coupling obtained from GPS measurements of upper plate deformation during the interseismic period are summarized. We compared those with new estimates of seismic coupling obtained from seismological data. The results show that with a few notable exceptions the two methods agree to within about 10%. The seismological estimates have been greatly improved over those made 20–30 years ago because of an abundance of paleoseismological data that greatly extend the temporal record of great subduction earthquakes and by the occurrence, in the intervening years, of an unusual number of great and giant earthquakes that have filled in some of the most critical holes in the seismic record. The data also, again with a few notable exceptions, support the frictional instability theory of seismic coupling, and in particular, the test of that theory made by Scholz and Campos (1995). Overall, the results support their prediction that high coupling occurs for subduction zones subjected to high normal forces with a switch to low coupling occurring fairly abruptly as the normal force decreases below a critical value. There is also considerable variation of coupling within individual subduction zones. Earthquake asperities correlate with areas of high coupling and hence have a semblance of permanence, but the rupture zones and asperity distributions of great earthquakes may differ greatly between seismic cycles because of differences in the phase of seismic flux accumulation.
The maximum earthquake magnitude recorded for subduction zone plate boundaries varies considerably on Earth, with some subduction zone segments producing giant subduction zone thrust earthquakes (e.g. Chile, Alaska, Sumatra–Andaman, Japan) and others producing relatively small earthquakes (e.g. Mariana, Scotia). Here we show how such variability might depend on various subduction zone parameters. We present 24 physical parameters that characterize these subduction zones in terms of their geometry, kinematics, geology and dynamics. We have investigated correlations between these parameters and the maximum recorded moment magnitude (MW) for subduction zone segments in the period 1900–June 2012. The investigations were done for one dataset using a geological subduction zone segmentation (44 segments) and for two datasets (rupture zone dataset and epicenter dataset) using a 200 km segmentation (241 segments). All linear correlations for the rupture zone dataset and the epicenter dataset (|R| = 0.00–0.30) and for the geological dataset (|R| = 0.02–0.51) are negligible-low, indicating that even for the highest correlation the best-fit regression line can only explain 26% of the variance. A comparative investigation of the observed ranges of the physical parameters for subduction segments with MW > 8.5 and the observed ranges for all subduction segments gives more useful insight into the spatial distribution of giant subduction thrust earthquakes. For segments with MW > 8.5 distinct (narrow) ranges are observed for several parameters, most notably the trench-normal overriding plate deformation rate (vOPD⊥, i.e. the relative velocity between forearc and stable far-field backarc), trench-normal absolute trench rollback velocity (vT⊥), subduction partitioning ratio (vSP⊥/vS⊥, the fraction of the subduction velocity that is accommodated by subducting plate motion), subduction thrust dip angle (δST), subduction thrust curvature (CST), and trench curvature angle (αT). The results indicate that MW > 8.5 subduction earthquakes occur for rapidly shortening to slowly extending overriding plates (−3.0 ⩽ vOPD⊥ ⩽ 2.3 cm/yr), slow trench velocities (−2.9 ⩽ vT⊥ ⩽ 2.8 cm/yr), moderate to high subduction partitioning ratios (vSP⊥/vS⊥ ⩽ 0.3–1.4), low subduction thrust dip angles (δST ⩽ 30°), low subduction thrust curvature (CST ⩽ 2.0 × 10−13 m−2) and low trench curvature angles (−6.3° ⩽ αT ⩽ 9.8°). Epicenters of giant earthquakes with MW > 8.5 only occur at trench segments bordering overriding plates that experience shortening or are neutral (vOPD⊥ ⩽ 0), suggesting that such earthquakes initiate at mechanically highly coupled segments of the subduction zone interface that have a relatively high normal stress (deviatoric compression) on the interface (i.e. a normal stress asperity). Notably, for the three largest recorded earthquakes (Chile 1960, Alaska 1964, Sumatra–Andaman 2004) the earthquake rupture propagated from a zone of compressive deviatoric normal stress on the subduction zone interface to a region of lower normal stress (neutral or deviatoric tension). Stress asperities should be seen separately from frictional asperities that result from a variation in friction coefficient along the subduction zone interface. We have developed a global map in which individual subduction zone segments have been ranked in terms of their predicted capability of generating a giant subduction zone earthquake (MW > 8.5) using the six most indicative subduction zone parameters (vOPD⊥, vT⊥, vSP⊥/vS⊥, δST, CST and αT). We identify a number of subduction zones and segments that rank highly, which implies a capability to generate MW > 8.5 earthquakes. These include Sunda, North Sulawesi, Hikurangi, Nankai-northern Ryukyu, Kamchatka-Kuril-Japan, Aleutians-Alaska, Cascadia, Mexico-Central America, South America, Lesser Antilles, western Hellenic and Makran. Several subduction segments have a low score, most notably Scotia, New Hebrides and Mariana.
We test hypothetical tsunami scenarios against a 4,600-year record of
sandy deposits in a southern Oregon coastal lake that offer minimum
inundation limits for prehistoric Cascadia tsunamis. Tsunami simulations
constrain coseismic slip estimates for the southern Cascadia megathrust
and contrast with slip deficits implied by earthquake recurrence
intervals from turbidite paleoseismology. We model the tsunamigenic
seafloor deformation using a three-dimensional elastic dislocation model
and test three Cascadia earthquake rupture scenarios: slip partitioned
to a splay fault; slip distributed symmetrically on the megathrust; and
slip skewed seaward. Numerical tsunami simulations use the hydrodynamic
finite element model, SELFE, that solves nonlinear shallow-water wave
equations on unstructured grids. Our simulations of the 1700 Cascadia
tsunami require >12-13 m of peak slip on the southern Cascadia
megathrust offshore southern Oregon. The simulations account for tidal
and shoreline variability and must crest the ˜6-m-high lake outlet
to satisfy geological evidence of inundation. Accumulating this slip
deficit requires ≥360-400 years at the plate convergence rate,
exceeding the 330-year span of two earthquake cycles preceding 1700.
Predecessors of the 1700 earthquake likely involved >8-9 m of
coseismic slip accrued over >260 years. Simple slip budgets
constrained by tsunami simulations allow an average of 5.2 m of slip per
event for 11 additional earthquakes inferred from the southern Cascadia
turbidite record. By comparison, slip deficits inferred from time
intervals separating earthquake-triggered turbidites are poor predictors
of coseismic slip because they meet geological constraints for only 4
out of 12 (˜33%) Cascadia tsunamis.
The 2004 Sumatra-Andaman earthquake of M
w 9.3 occurred in a region where a giant earthquake seemed unlikely from the point of view of tectonics. This clearly implies that our current understanding of strain accumulation processes of large earthquakes at subduction zones needs to be reexamined. The Ryukyu subduction zone is one such zone since no large earthquake has been anticipated there for reasons similar those pertaining to the Sumatra-Andaman arc. Based on our analysis of historical earthquakes, plate motion, back-arc spreading, and GPS observation along the Ryukyu trench, we highly recommend monitoring seafloor crustal deformation along this trench to clarify whether a large earthquake (M
w>8) could potentially occur there in the future.
Immediately following the 11th March 2011 Mw 9.0 Tohoku (Japan) earthquake, a field investigation was carried out around the Tokyo Bay area. This paper provides first-hand observations (before or just at the onset of repair) of widespread liquefaction and the associated effects. Observations related to uplift of manholes, settlement of ground, performance of buildings and bridges and the effects of ground improvements are also presented. Recorded ground motions near the Tokyo Bay area were analysed to understand their key characteristics (large amplitude and long duration). Lessons learnt are also presented.Highlights► Geotechnical field investigations of the 2011 Mw 9.0 Tohoku earthquake are reported. ► First-hand observations of widespread liquefaction around the Tokyo Bay area are presented. ► The effects of long-duration ground motions critical to liquefaction-related damage are analysed. ► Successful cases of ground improvement for reclaimed/filled land are discussed.
We investigate angular velocity vectors of the Philippine Sea (PH) plate relative to the adjacent major plates, Eurasia (EU) and Pacific (PA), and the smaller Caroline (CR) plate. Earthquake slip vector data along the Philippine Sea plate are inverted, subject to the constraint that EU-PA motion equals that predicted by the global relative plate model NUVEL-1. The resulting solution fails to satisfy geological constraints along the Caroline-Pacific boundary: convergence along the Mussau Trench and divergence along the Sorol Trough. We then seek solutions satisfying both the CR-PA boundary conditions and the Philippine Sea slip vector data, by adjusting the PA-PH and EU-PH best fitting poles within their error ellipses. We also consider northern Honshu to be part of the North American plate and impose the constraint that the Philippine Sea plate subducts beneath northern Honshu along the Sagmi Trough in a NNW-NW direction. Of the solutions satisfying these conditions, we select the best EU-PH as 48.2 deg N, 157.0 deg E, 1.09 deg/my, corresponding to a pole far from Japan and south of Kamchatka, and PA-PH, 1.2 deg N, 134.2 deg E, 1.00 deg/my. Predicted NA-PH and EU-PH convergence rates in central Honshu are consistent with estimated seismic slip rates. Previous estimates of the EU-PH pole close to central Honshu are inconsistent with extension within the Bonin backarc implied by earthquake slip vectors and NNW-NW convergence of the Bonin forearc at the Sagami Trough.
Located near Japan's most densely populated and industrially active region, the Nankai Trough subduction zone has long been highlighted as a high-risk area for damaging earthquakes and tsunamis. In contrast, less attention has been paid the adjacent Ryukyu Trench because historical and geological records are scarce. In order to develop better quantitative estimates of the timing and size of the earthquakes and tsunamis generated along these subduction zones, comprehensive studies using geological, seismological and historical methods have been conducted. Since the 1990s, studies of tsunami deposits in this region have contributed to our current understanding of the history of tsunamis over the last 6000 years. Following the 2011 Tōhoku earthquake and tsunami, paleotsunami research has especially focused on guiding and enhancing tsunami disaster management and mitigation measures. The last nine years have seen a rapid increase in paleotsunami data from the Nankai Trough and Ryukyu Trench coasts. These recent studies reveal that there are significant differences in the size and recurrence pattern of earthquakes and tsunamis along these subduction zones. For instance, large earthquakes have repeatedly occurred along the Nankai Trough every 100–200 years. On the other hand, large earthquakes and tsunamis may not have occurred over the last few thousand years at the junction between these subduction zones. There is also no evidence for the occurrence of large earthquakes along the northern and central Ryukyu Trench over this time period. Large earthquakes and tsunamis may have occurred every few hundred years at the southern end of the Ryukyu Trench. Nevertheless, data on the occurrence of these earthquakes are still insufficient, both in quantity and quality, to estimate the maximum size or recurrence pattern.
Coastal boulders potentially provide very useful information to reconstruct hydraulic characteristics of extreme waves such as tsunamis or storm waves that struck shores in historical and prehistoric eras. Boulder transport models, which are strong tools to reconstruct the hydraulic characteristics during boulder transport, can be classified into inverse and forward models for identification and size estimation of tsunami or storm wave boulders. An inverse model can estimate the minimum wave height necessary to move a boulder and the minimum wave velocity necessary to slide, rotate, or saltate the boulder. A forward model can estimate precise hydraulic parameters such as the maximum wave velocity or wave runup height by reproducing the boulder transport distance. While some models are useful for practical purposes, few parameters are included in these models because they have been developed by simplification of actual phenomena. Therefore, it is noteworthy that hydraulic parameters estimated from the boulder transport model still include large error and uncertainty: the models need to be improved. Future work must be conducted to estimate the tsunami source or the storm size based on the tsunami or storm wave boulder distribution. Such estimation results are expected to be useful for coastal risk assessments.
We performed observations of seafloor crustal deformation employing the Global Navigation Satellite System/Acoustic technique at two stations installed at about 45 and 70 km from the axis of the Nansei-Shoto (Ryukyu) Trench, to the southeast off the Okinawa Main Island. The observations for 3- and 6-year survey periods indicate that the two stations moved landward, in the opposite direction to the trench, by 63 and 21 mm/year relative to the Ryukyu Arc, suggesting interplate coupling around the stations. The observational results reveal the strong coupling state on the plate interface with coupling ratios of 0.9-1.0 at the slab depths of 10-13 km or 0.7-0.8 at slab depth ranging from 13 km up to the seafloor. The strongly coupled segment completely coincides with the source area of the 1791 tsunami event and does not overlap with the activity area of slow slip events.
Along the Ryukyu Islands, 11 tsunamis have been recorded since 1664. Among them, there was the 1771 Yaeyama tsunami which hit the Ishigaki and Miyako Islands. About 12, 000 persons were drowned and 3, 200 houses washed away. In this paper, the mangnitudes of the principal tsunamis are discussed by the author's method (HATORI, 1986), using the diagram of attenuation of tsunami height with distance. Refraction diagrams of representative tsunamis are drawn to see travel times and the shoaling effects for each island.
The run-up heights at Ishigaki and Miyako Islands facing the tsunami source reached 20-30 meters, while those at the coasts of the East China Sea side decreased to 4-5 meters. And the run-up heights at Taketomi and Iriomote Islands etc. laying the wide coral reefs conspicuously decreased, too, suggesting that the short-period waves were predominant. The source area extends 100km along the mean depth 2, 500m which the location is expected with the wave preiod 10min. The wave rays projected from the half margin of source area concentrate between Miyako and Iriomote Islands. The wave rays of the hypothetical sources for the 1911 Amami-Oshima and 1938 Miyakojima tsunamis also concentrate within the segment of about 200km. Judging from the tsunami height-distance diagram, the magnitude of Imamura-Iida scale for the 1771 Yaeyama tsunami was m=4, but that of the Amami-Oshima tsunamis in 1901 and 1911 was determined to be m=1 and m=1.5, respectively, which are somewhat larger than the former values.
Tsunamis do not take place so frequently in the region of the Ryukyu Islands as that around the mainland of Japan, but those of the various types have been generated by the low-frequency or deep earthquakes. Most of historical small or moderate tsunamis may be missed, comparing with the recent seismic activity. In a map of the source distribution, there are manygaps of tsunami source area.
This paper presents data and analysis for block and boulder transport during Super Typhoon Haiyan along a 4.5km long, low (5-12m) cliffed coastline in Calicoan Island, Eastern Samar, Philippines. Wave runup exceeding 15.2m elevation above mean sea level drove large limestone clasts, with volumes up to ~ 83m³, up to ~ 280m inland. A few very large clasts with volumes 65-132m³ were not transported by the waves. When combined with recent transport reported in May et al. (2015), Cox et al. (2016), and other literature, it is becoming increasingly clear that the largest blocks transported by storms overlie much of the tsunami transport range, increasing the difficulty in attributing the transport source without additional evidence. Comparison of present results with a global database of storm boulder transport shows a mass-elevation envelope outside of which no transport is observed.
Holocene coral reefs are formed over equatorial pressure belts, trade wind belts, and subtropical high-pressure belts (between latitude 30° N and 30° S). In equatorial pressure belt and trade wind belt regions, few coral reefs are affected by storm-induced waves. In subtropical high-pressure belts, on the contrary, sediments on coral reefs are transported by catastrophic waves generated by violent storms (typhoons, hurricanes and cyclones). The wind belts could influence the geomorphic processes of coral reefs, resulting in different landforms. Several typhoons hit the Ryukyu Islands located in southwestern Japan every summer, and their coral reefs have sustained successive damages before recovering. Large blocks (more than 1 m in longest diameter) detached from reef slopes are found on reef flats in some coral reefs. They can move during catastrophic disturbance, and may be transported by storm-induced waves. The object of this study is therefore to evaluate the effect of large reef blocks on geomorphic formation in the typhoon area.
Around Kudaka Island in the Ryukyu Islands, fringed coral reefs and the distribution of large reef blocks were investigated by aerial photo interpretation. Forty-one large reef blocks located on the reef flat and moat were measured and characterized. Movements of large reef blocks on them were observed from October 1994 to September 1996. Coral clasts, which were randomly collected from the moat, were placed on the reef flat under calm sea conditions and the direction and distance of movements were measured.
Typhoon-induced waves swept across the reef edge and large blocks were transported from the reef slope to the reef flat. These blocks are classified as “groove blocks” and “spur blocks” based on their shape. The spur blocks, which originated from the reef framework and dead corals, are angular, while groove blocks are rounded as a result of abrasion by spur blocks. Spurs developed sideways during storms, and reef blocks were detached from spurs. Through aerial photo interpretation and field observations, it was determined that reef blocks on the reef flat were transported at intervals of a few decades. These movements are much quicker and the production of reef blocks much more frequent in the trade wind zone where blocks are transported on a geological time scale.
Movements of blocks were observed during the winter seasonal wind and typhoons of 1996. In the summer of 1996, reef blocks were thrown up over 50m from the reef slope on the reef flat by the wave forces of typhoons with wind speed of 20m/s or more and maximum significant wave height of 6 m or more. Only groove blocks and live corals were transported and no movement of spur blocks was observed. Horizontal movements of large blocks were recorded for distances of over 50m and vertical movements of blocks were recorded for distances of over 5m during typhoons. Spur blocks would be transported by waves of typhoons bigger than those that occurred during this investigation. Reef slopes in the Ryukyu Islands are gentle, and therefore it is easy for reef boulders to be thrown up from reef slopes to reef flats.
Most of blocks on the reef flat and the moat are characterized by depressions of erosion that suggest the blocks have been exposed to the air. The scattered reef blocks show a heterogeneous distribution in convex reef flats. This suggests that large blocks have been transported by wave processes. Blocks in the moat become live coral basements, and some blocks become cemented to the coral, causing the moat to pile up.
The 2011 Tohoku Earthquake which exceeds the maximum magnitude of historical earthquake records could not be predicted. Which means, in the probabilistic tsunami hazard assessment for the future prediction, it is necessary to apply a different concept from the existing scenario tsunami assessment based on the maximum magnitude of historical earthquake tsunami. As a new modeling method for scenario tsunami, we propose a characterized tsunami source model which indicates setting method of the area of tsunami source and slip distribution by the inter-plate earthquake.
The occurrence of large earthquakes and tsunamis along the Ryukyu Trench is a subject of continuing interest, the key to which is the long-term geological record. Here we describe the clast size and spatial distributions of similar to 2900 boulders on the reefs of the Ryukyu Islands, Japan, as markers of paleotsunamis and causative tsunamigenic earthquakes. Boulders of tsunami origin were observed only at a specific island group at the southern end, suggesting the local occurrence of tsunamigenic earthquakes there. In contrast, in the central to northern Ryukyu Islands, no evidence exists of tsunamis larger than those at the southern end of the Ryukyu Islands during the past 2000-3000 yr. These islands have numerous boulders deposited by storm waves during the past 2300 yr or earlier. Their spatial distribution has not been disturbed by large tsunamis. This suggests that large tsunamis did not strike this area during that period; nevertheless, these regions are seismically active. Our study shows that coastal boulder deposits present great potential to not only ascertain the histories and effects of paleotsunamis but also to constrain fault models of the causative earthquakes.
Long-term activity of shallow very low frequency earthquakes (VLFEs) in the Ryukyu Trench, unassociated with recent large thrust earthquakes, was analyzed using a broadband seismometer network. The distribution of shallow VLFEs was divided into three large clusters. The activity of the VLFEs is modulated by repetitive slow slip events and the activity of these VLFEs increases to 2–3 times its ordinary rate at 10–20 days after the onset of the slow slip events. The activation of VLFEs could be generated by small increases in the order of approximately 0.1–0.5 kPa in Coulomb failure stress, suggesting that stress in the plate interface where the VLFEs occur is frequently released in small amounts. Moreover, the distribution of VLFEs is complementary to the historical tsunami source area and locked area. The distribution of the VLFEs indicates heterogeneity in interplate coupling along the trench.
 The rupture parameters and magnitude of the AD 869 Jogan earthquake, a predecessor of the 2011 Tohoku earthquake, were previously estimated by matching tsunami deposit distributions with simulated inundation areas. The tsunami inundation associated with the 2011 Tohoku earthquake, however, extended farther inland than the sandy tsunami deposits. Numerical simulation of the 2011 tsunami indicated that flow depths and velocities were approximately 1 m and 0.6 m/s, respectively, at the most inland sand deposit sites on the Ishinomaki and Sendai plains. While these values depend on the assumed bottom roughness, we used these values to compare tsunami deposits and inundation simulation of the 869 Jogan earthquake from both uniform-slip and 2011-type variable-slip fault models. The results showed that the rupture length of the 869 Jogan earthquake was at least 200 km and its minimum moment magnitude was 8.6.
We have conducted hydraulic experiments in an open channel with cubic
and rectangular shaped solid blocks on the slope for investigating the
boulder transport process by tsunami. In our experiments, the block was
mainly seen to be transported by a bore due to rolling or saltation
rather than by sliding. Previous models for the boulder transport by
tsunamis assumed sliding as a mode of transport for the boulder.
Therefore, these models underestimated the distance of the boulder moved
by the tsunami when it was transported due to rolling or saltation. In
this study, we have developed a practical model for the transport of a
boulder by tsunami, which takes into account the various transport
modes. We introduce an empirical variable coefficient of friction by
assuming that the coefficient decreases with decrease in ground contact
time when the block was transported by rolling or saltation. With the
aid of this parameter, the model can explain various modes of transport,
i.e., sliding, rolling, and saltation, and reproduces the experimental
results well. We further applied this improved model to a tsunami
boulder at Inoda area in Ishigaki Island, Japan, which was transported
by the 1771 Meiwa tsunami. The calculated distance of transport of the
boulder was approximately 650 m, which is consistent with the
description in the historical document. Based on our calculations, we
estimated hydraulic values of the tsunamis. Estimation of such hydraulic
values is important for understanding the behavior and power of the
historical tsunamis, besides aiding future disaster mitigation efforts.
The study of palaeotsunamis preserved in the sedimentary record has developed over the past three decades to a point where the criteria used to identify these events range from well-tested and accepted to new methods yet to receive wide application. In this paper we review progress with the development of these criteria and identify opportunities for refinements and for extending their application to new settings. The emphasis here is on promoting the use of multiple proxies, selected to best match the context of the site or region of interest. Ultimately, this requires that palaeotsunami research must be a multidisciplinary endeavour and indeed, extend beyond the geological sciences of sedimentology and stratigraphy and, to include knowledge and approaches from field such as archaeology, anthropology and sociology. We also argue that in some instances, despite the use of multiple proxies, the evidence for tsunami inundation of a coast simply may not be preserved.
On 29 September 2009 a large tsunami struck the US territory of American Samoa, killing 34 people. However, had it not been for the island's tsunami preparation, the death toll would have been much higher. An integral part of preparing for a tsunami is knowing how often large tsunamis occur, knowledge that can be improved by using paleo-tsunami sediment deposits to lengthen the historical tsunami record. Doing so requires a detailed understanding of the processes that control sediment transport during tsunamis and thus the patterns of deposition, which may vary substantially in sediment limited and sediment rich environments. As American Samoa is relatively young and composed of volcanic islands surrounded by coral reefs, sandy beaches occur primarily as pocket beaches between volcanic rock headlands. The extent and depth of littoral sediment available for transport is therefore limited. This is in contrast to previous studies which have focused primarily on coastal environments with extensive sand supply. Here, field observations and a numerical model are used to investigate tsunami-induced sediment transport in two sediment limited embayments (Massacre and Fagafue Bays) on the north side of the island of Tutuila in American Samoa. Detailed measurements of bathymetry, topography, tsunami flow depth and direction, sediment deposition, reef and vegetative roughness, and the extent of sediment available for transport were collected approximately two weeks after the tsunami. Both embayments contain sandy sediments that extend all the way (Massacre Bay) and part of the way (Fagafue Bay) along the beach at the head of the embayment. The observed onshore sediment deposit created by the tsunami was limited and patchy in both embayments even though a number of large coral boulders were transported onshore at both locations. The extent to which the limited supply of sediment plays in producing the patterns of deposition observed in these two bays is examined using a three-dimensional coupled hydrodynamic/sediment transport/morphological change model (Delft3D). To isolate this effect, model simulations examine the effects on sediment deposition of wave focusing owing to embayment shape and reef channels, and the strong return flow generated by the steep onshore topography. Differences between the depositional patterns in sediment limited and sediment rich environments are identified by comparing the results from these simulations with results from simulations where the extent and depth of the sediment source are increased and from simulations conducted in a sediment rich environment (Kuala Meurisi, Sumatra).
Field observations and numerical simulations are used to explore tsunami inundation and sediment transport in an embayment (Fagafue Bay) on the north side of Tutuila, American Samoa during the 29 September 2009 South Pacific tsunami. Field observations of the nearshore bathymetry and topography, tsunami flow depth and sediment deposition, and extent of movable sandy sediment remaining on the beach were collected during two field surveys approximately two and five weeks after the tsunami. Onshore measurements of flow depth at forty-eight locations indicate the wave inundated almost 250m onshore with a depth exceeding 7m locally. The tsunami deposited patchy areas of sediment up to 0.2m thick interspersed with a thin dusting (
The 24 April 1771 Yaeyama earthquake generated a large tsunami with a maximum runup of 30 m, causing significant damage in south Ryukyu, Japan, despite the weak ground shaking. Previously proposed mechanisms of the tsunami include intraplate faulting or submarine landslide in the forearc slope. In this study, I estimate the fault parameters of the 1771 earthquake by numerically computing the tsunami heights and comparing them with the recorded heights. The result indicates that the source fault of the tsunami is very close to the Ryukyu Trench. The results are consistent with a thrust-faulting earthquake that had a fault-width of less than 50 km. The 1771 Yaeyama tsunami was caused by a tsunami earthquake (Mw = 8.0) that occurred in the subducted sediments beneath the accretionary wedge.
The amount of shallow seismic activity in subduction zones varies greatly from region to region. This paper quantifies this seismicity by calculating seismic moment release rates and seismic slip rates for 24 subduction zones. A time history plot of the seismic moment release is presented for each subduction zone; these exhibit the differences in the seismic release patterns. The moment release rates are compared with various subduction parameters in order to determine which factors influence the degree of coupling. These parameters include the age of the subducting lithosphere, absolute velocities of the upper and subducting plates, convergence velocity, and length maximum depth, and dip of the Wadati-Benioff zone. The moment release rate decreases as the age of the subducting lithosphere increases, when the zones belonging to a single subducting plate are considered. It is suggested that within a single plate, the age is the dominating factor affecting the strength of seismic coupling but that each plate as a whole has a characteristic moment release budget. Zones with retreating upper plates tend to have lower moment release rates. The moment release rate does not increase with convergence velocity; no simple relationship was found between these two parameters.
Cores from Cascadia deep-sea channel contain sequences of turbidites
that can be correlated and dated by the first occurrence of volcanic
glass from the Mount Mazama eruption (6845±50 radiocarbon yrBP).
Turbidity currents from the tributaries appear to have occurred
synchronously to form single deposits in the main channel, there being
only 13 turbidite deposits in the lower main channel since the Mazama
eruption, instead of the twice as many expected if the tributaries had
behaved independently. In addition to the Cascadia Channel, 13
post-Mazama turbidites have been deposited in the Astoria Canyon and at
two sites off Cape Blanco, sample locations that span 580 km of the
Oregon-Washington margin. Pelagic intervals deposited between the
turbidites suggest that in each place the turbidity currents occurred
fairly regularly, every 590±170 years on average. The best
explanation of the spatial and temporal extent of the data is that the
turbidity currents were triggered by 13 great earthquakes on the
Cascadia subduction zone. The variability of turbidite timing is similar
to that for great earthquake cycles. The thickness of the topmost
pelagic layer suggests the last event was 300±60 years ago (from
three places along the margin), but this number may be a biased
underestimate. It is, however, consistent with the youngest
sudden-subsidence event on the Washington coast. The turbidite data
demonstrate that the near-term hazard of a great earthquake on the
Cascadia subduction zone is of the order of 2-10% in the next 50 years.
Case studies of recent tsunami impacts have proven to be extremely useful in understanding the geologic processes involved during inundation and return flow, and refining the criteria used to identify paleotsunami deposits in the geologic record. Here, we report on erosion, deposition and associated landscape change resulting from the March 11, 2011 Tohoku-oki tsunami along a nearly 4.5 km shore-normal transect on the coastal plain near Sendai, Japan. The study area on the broad, low-relief prograding coastal Sendai plain comprised a sand beach backed by ~ 3 m high sand dunes and a forest, a wetland, the Teizan canal, agricultural rice fields, buildings and roads.
We report initial results from our recent field survey documenting the inundation and resultant deposits of the 2011 Tohoku-oki tsunami from Sendai Plain, Japan. The tsunami inundated up to 4.5 km inland but the > 0.5 cm-thick sand deposit extended only 2.8 km (62% of the inundation distance). The deposit however continued as a mud layer to the inundation limit. The mud deposit contained high concentrations of water-leachable chloride and we conclude that geochemical markers and microfossil data may prove to be useful in identifying the maximum inundation limit of paleotsunamis that could extend well beyond any preserved sand layer. Our newly acquired data on the 2011 event suggest that previous estimates of paleotsunamis (e.g. 869 AD Jōgan earthquake and tsunami) in this area have probably been underestimated. If the 2011 and 869 AD events are indeed comparable, the risk from these natural hazards in Japan is much greater than previously recognized.
Keywords: Tsunami deposits; Tohoku-oki tsunami; Japan
Abstract The taxonomic diversity of hermatypic corals decreases with increasing latitude, which correlates with sea-surface temperatures. However, little is known about latitudinal changes in the taxonomic diversity and biogeographic patterns of larger benthic foraminifera, although their physiological requirements are similar to those of hermatypic corals because of their symbiotic relationships with microalgae. The present study examined how the abundance and taxonomic composition of larger foraminiferal assemblages in shallow-water reef sediments change with latitude along the Ryukyu Islands (Ryukyus), which are located near the northern limit of coral-reef distributions in the western Pacific Ocean. Three islands from different latitudes in the Ryukyus were selected to investigate latitudinal changes in larger foraminiferal assemblages: Ishigaki Island (24°20′N, 124°10′E), Kudaka Island (26°09′N, 127°54′E) and Tane-ga-shima Island (30°20′N, 131°E). Four sediment samples were taken at each of three topographic sites (beach, shallow lagoon and reef crest) on the reef flat of each island. Foraminiferal tests of a 2.0- to 0.5-mm size fraction were selected, identified and counted. The variations in foraminiferal abundance in reef sediments from three latitudinally different islands exhibit two contrasting trends along reef flats: a shoreward decrease on Ishigaki and Tane-ga-shima Islands and a shoreward increase on Kudaka Island. A total of 25, 24 and 13 foraminiferal taxa were identified in Ishigaki, Kudaka and Tane-ga-shima Islands, respectively. Baculogypsina sphaerulata, Neorotalia calcar and Amphistegina spp. were dominant (i.e. >3% of foraminiferal assemblages) in the three islands. Calcarina gaudichaudii and Calcarina hispida were common on Ishigaki and Kudaka Islands but were absent on Tane-ga-shima Island. Larger foraminiferal assemblages from three different reef-flat environments on Ishigaki Island can be distinguished, whereas those from the three environments on Kudaka and Tane-ga-shima Islands are similar in composition. These latitudinal changes in larger foraminiferal assemblages in reef sediments may possibly be caused by variations in the topography of reef flats, distributions and standing crops of living foraminifers on reef flats, and the northern limit of some calcarinid species in the northern Ryukyus.
Tsunamis are high energy events capable of transporting extremely heavy loads including boulders. We compare boulder deposits created by two modern tsunami events, the 2004 Indian Ocean and the 2009 South Pacific tsunamis, where the boulder sources were in similar topographic settings, and for which we have accurate data on the wave characteristics. Boulder distribution, preferential orientation and numerical simulation of boulder transport are discussed. A comparison between the impacts of the South Pacific and Indian Ocean tsunamis shows similar characteristics, such as limited landward extent and the absence of landward fining. Differences between the results from modelling and field data are most probably caused by variables such as coastal plain roughness (buildings, trees), microtopography, particle shape, and boulder collision during transport that are summarised as coefficients in the mathematical models. Characterising modern events through coarse sediment deposits provides valuable information to help identify and interpret palaeo-tsunami imprints on coastal landscapes.
A combination of numeric hydrodynamic models, a large-clast inverse sediment-transport model, and extensive field measurements
were used to discriminate between a tsunami and a storm striking Anegada, BVI a few centuries ago. In total, 161 cobbles and
boulders were measured ranging from 1.5 to 830kg at distances of up to 1km from the shoreline and 2km from the crest of
a fringing coral reef. Transported clasts are composed of low porosity limestone and were derived from outcrops in the low
lying interior of Anegada. Estimates of the near-bed flow velocities required to transport the observed boulders were calculated
using a simple sediment-transport model, which accounts for fluid drag, inertia, buoyancy, and lift forces on boulders and
includes both sliding and overturning transport mechanisms. Estimated near-bed flow velocities are converted to depth-averaged
velocities using a linear eddy viscosity model and compared with water level and depth-averaged velocity time series from
high-resolution coastal inundation models. Coastal inundation models simulate overwash by the storm surge and waves of a category
5 hurricane and tsunamis from a Lisbon earthquake of M 9.0 and two hypothetical earthquakes along the North America Caribbean
Plate boundary. A modeled category 5 hurricane and three simulated tsunamis were all capable of inundating the boulder fields
and transporting a portion of the observed clasts, but only an earthquake of M 8.0 on a normal fault of the outer rise along
the Puerto Rico Trench was found to be capable of transporting the largest clasts at their current locations. Model results
show that while both storm waves and tsunamis are capable of generating velocities and temporal acceleration necessary to
transport large boulders near the reef crest, attenuation of wave energy due to wave breaking and bottom friction limits the
capacity of storm waves to transport large clast at great inland distances. Through sensitivity analysis, we show that even
when using coefficients in the sediment-transport model which yield the lowest estimated minimum velocities for boulder transport,
storm waves from a category 5 hurricane are not capable of transporting the largest boulders in the interior of Anegada. Because
of the uncertainties in the modeling approach, extensive sensitivity analyses are included and limitations are discussed.
KeywordsBoulder–Tsunami–Hurricane–Cyclone–British Virgin Islands–Caribbean
This study investigates the size, position and the long axis orientation of 210 boulders at Kudaka Island, Japan. These boulders were deposited from the reef crest to the slope of the back reef moat, distributed within 275 m from the reef edge. Most boulders were rectangular to ellipsoidal, without sharp broken edges. They are reef rock fragments estimated as < 63 m3 (< 127 t). The second largest boulder (54 t) was not observed in aerial photographs taken in 1977 and 1993, although it appears in photographs taken in 2005 and 2007. Considering that no large tsunami event occurred during 1993–2005, the second largest boulder is expected to have been emplaced by typhoon-generated storm waves. Moreover, the positions of many boulders were found to have shifted after 1977. These boulders were highly likely to have been repositioned by the storm waves. Results showed that boulders' motion follows an exponential fining trend shoreward. This trend fits well with the distribution of the height of the storm wave after breaking on the reef flat. The largest storm waves after 1977 (typhoon 0704 in 2007) were probably responsible for the current boulder distribution. Using the relation between the distributions of boulders and the significant wave height of typhoon 0704, the approximate transport distance of boulders by an arbitrary storm wave at the island can be estimated. The storm wave boulders' distribution is also useful to estimate the storm wave properties: we estimated the maximum current velocity distribution of waves generated by typhoon 0704 on the reef flat as up to 6.5 m/s using the boulder distribution.
Coastal boulder accumulations are often mentioned in the literature, even though their interpretation remains difficult, especially along rock coasts affected both by storms and tsunamis. Studies on the geomorphic impact of such high-energy events are actually of great interest, since their intensity and frequency are key issues for the future evolution of coasts in the framework of the global change. The southwest coast of Iceland faces the powerful storms of the North Atlantic Ocean, with wave heights of more than 15 m. The probability for past and present tsunamis to hit this coast is very low. In this paper, we describe boulder accumulations along the volcanic rock coast of Reykjanes (southwest Iceland). They consist of cliff-top boulders, clusters and ridges, beaches, and boulder fields. Large boulders, up to 70 t in weight, have been transported and deposited up to 65 m inland (6 masl). The maximum limit of boulder deposition and driftwood was found respectively 210 m and 550 m inland. Storms appear to be a predominant factor in the geomorphic evolution of Reykjanes coasts. Our observations also give new insight for the interpretation of coastal boulder accumulations. Processes of erosion and deposition by tsunamis are a rising topic in the literature, and the effects of recurrent and powerful storms are neglected.
Using a GPS derived velocity field, we try to estimate the kinematic features of the Ryukyu arc, and discuss its implications to the spreading of the Okinawa trough. The GPS derived strain rate field implies that internal deformation within the Ryukyu arc is not significant, therefore, we adopt a rigid block rotation model for the motion of the Ryukyu arc. The velocity field in the Ryukyu arc is best explained by a model with three blocks rather than one with only one block. The estimated block rotations predict the spreading along the Okinawa trough. The predicted spreading is slow (10 mm per year) in the north and fast (50 mm per year) in the southwest. The direction of the predicted spreading is normal to the Okinawa trough (N170°E–N180°E) in the southwest, while in the north the direction of the spreading is oblique to the Okinawa trough (N150°E–N180°E).
The tsunami of 2004 in the Indian Ocean transported thousands of meters-long boulders shoreward at Pakarang Cape, Thailand. We investigated size, position and long axis orientation of 467 boulders at the cape. Most of boulders found at the cape are well rounded, ellipsoid in shape, without sharp broken edges. They were fragments of reef rocks and their sizes were estimated to be < 14m3 (22.7t). The distribution pattern and orientation of long axis of boulders reflect the inundation pattern and behavior of the tsunami waves. It was found that there is no clear evidence indicating monotonous fine/coarse shoreward trends of these boulders along each transect line. On the other hand, the large boulders were deposited repeatedly along the three arcuate lines at the intertidal zone with a spacing of approximately 136m interval. This distribution pattern may suggest that long-lasting oscillatory flows might have repositioned the boulders and separated the big ones from small. No boulders were found on land, indicating that the hydraulic force of the tsunami wave rapidly dissipated on reaching the land due to the higher bottom friction and the presence of a steep slope. We further conducted numerical calculation of tsunami inundation at Pakarang Cape. According to the calculation, the sea receded and the major part of the tidal bench (area with boulders at present) was exposed above the sea surface before the arrival of the first tsunami wave. The first tsunami wave arrived at the cape from west to east at approximately 130min after the tsunami generation, and then inundated inlands. Our calculation shows that tsunami wave was focused around the offshore by a small cove at the reef edge and spread afterwards in a fan-like shape on the tidal bench. The critical wave velocities necessary to move the largest and average-size boulders by sliding can be estimated to be approximately 3.2 and 2.0m/s, respectively. The numerical result indicates that the maximum current velocity of the first tsunami wave was estimated to be from 8 to 15m/s between the reef edge and approximately 500m further offshore. This range is large enough for moving even the largest boulder shoreward. These suggest that the tsunami waves that were directed eastward, struck the reef rocks and coral colonies, originally located on the shallow sea bottom near the reef edge, and detached and transported the boulders shoreward.
Sedimentary features and identification criteria of boulders deposited by tsunamis and storm waves are highly controversial because of the lack of detailed studies of boulders that are known to have been deposited by tsunami or storm waves. The coastal boulder fields of the Ryukyu Islands, Japan are one of the few places where comparisons can be made between the distribution and characteristics of boulders deposited by a known historical tsunami and storm waves. The 1771 Meiwa Tsunami struck the southern Ryukyu Islands (Miyako–Yaeyama Islands) and reliable historical documents describe run-up heights of up to 30 m. The displacement of specific boulders by the tsunami is also described in detail. Some of the islands away from the Miyako–Yaeyama Islands were unaffected by this tsunami, but they have been extensively affected by typhoon-generated storm waves. On these islands, the boulders were commonly deposited on the reef flat within 300 m of the reef edge as an exponentially fining landward deposit. This provides a useful indication of the transport limit for storm waves on the Ryukyu Islands. In the tsunami-affected islands, boulders of different types have been deposited both on the reef crest and along the shoreline. The reef crest boulders are identified as storm wave emplaced, whereas those along the shoreline are interpreted as tsunami boulders (“tsunami-ishi” in Japanese) because they are exceedingly heavy and are deposited well beyond (ca. 1.5 km from the reef edge) the transport limit for storm waves. Their 1771 Meiwa Tsunami origin is supported by 14C age results, although prior tsunami(s) may have deposited some of the boulders. Based on these results, we infer that the difference between the wave periods of tsunami and storm waves is crucial to differentiating tsunami boulders from other enigmatic boulder deposits around the world. Differences in wave period are reflected in differences between the spatial and clast size distributions of boulder deposits. The distribution and sedimentary characteristics of tsunami boulders therefore provide useful data for estimating possible tsunami sources. The boulders on the Ryukyu Islands are also useful for differentiating between tsunami and storm wave emplacement and for estimating their hydrodynamic properties.
Few studies have been conducted on modeling boulder transport by tsunamis despite considerable research on the analysis of boulder deposits. A detailed description of the derivation of governing equations for boulder transport in submerged, partially submerged, and subaerial (not in contact with fluid) is presented, and then a numerical model is proposed to solve the governing equations in one dimension. Subsequently, the model is used to analyze the transport of calcareous boulders detached from a seawall in Lhok Nga (northwestern Sumatra, Indonesia) by the 2004 Indian Ocean tsunami. A few simulated transport distances match field observations, but the others are higher than the field measurements. Clast-to-clast interactions at the inception of transport would have a major impact on changes in transport distance, dissipating the energy in impulses as destruction of the seawall releases different sizes of boulders with different velocities. Moreover, surface microtopographical effects could completely stop the transport prematurely. The difference between the simulated results and the field observations is partly attributed to limitations of the numerical model. No landward fining was observed in the field measurements, but numerically predicted results showed a reasonable trend of landward fining.
Invasion of about 3400 cal BP large wave in the southeastern Okinawa Island and the surroundings, the Ryukyus, Japan, as deduced from coralline deposits
Kawana, T., 2006. Invasion of about 3400 cal BP large wave in the southeastern
Okinawa Island and the surroundings, the Ryukyus, Japan, as deduced from
coralline deposits. Ryukyu Univ. Kyouikugaku Bukiyo 68, 265-271 (in Japanese).
Estimation of the historical large wave based on the position of coral boulders on the reef
Formative history of coral reef and earthquake and tsunami at Ryukyu Islands
Kawana, T., 2011. Formative history of coral reef and earthquake and tsunami at
Ryukyu Islands. Ryukyu Islands Dur. Prehist. Hist. Age, 63-86 (in Japanese).